Method for viscose production

ABSTRACT

A process for producing viscose wherein the caustic soda used previously in the alkalization of electron-untreated cellulose can be used in the alkalization of electron-treated cellulosic material, provided the level of dispersed solids in the caustic soda does not exceed 0.16 g/l.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved process for producingviscose.

2. Description of Related Art

The production of viscose from cellulosic materials is well known. Anextensive description of this technology may be found, for example, inK. Götze, Chemiefasern nach dem Viskoseverfahren, Springer-Verlag, 3rdedition (1967) and in the chapter “Viscosereyon, -spinnfasern,-schwämme” in Ullman's Enzyklopädie der technischen Chemie, Volume 18,pages 131-177, and Volume 9, pages 213-222.

The individual steps of the viscose process can briefly be summarized asfollows:

Chemical pulp or some other suitable cellulosic raw material is treatedwith an aqueous solution of caustic soda whose concentration iscustomarily between 18 and 22% to obtain alkali cellulose. Today, thealkalization is predominantly carried out by steeping. The cellulosicraw material is dispersed in the aqueous caustic soda solution in avessel or vessel system equipped with suitable dispersing means. Alkalicellulose forms. The dispersion formed is frequently also referred to asthe steep and generally has a solids content of 2-6%. The excess liquidis removed in roll or belt presses to isolate the alkali celluloseformed. Alkali cellulose comprises about 30-36% of cellulosic material,about 15-20% of NaOH and water. The removed caustic soda is thenrecycled into the steeping step. The addition of fresh caustic soda andthe removal of a bleed stream maintain the composition in this causticrecycle system consistently at the desired values.

The isolated alkali cellulose is aged, i.e., exposed to the action ofair, to achieve some oxidative degradation which depolymerizes thecellulose molecules to a suitable chain length for the rest of theprocess. This aging step requires a residence time of up to 1.5 days.

The foregoing alkalization of the cellulose can also be effected bysteeping electron-treated cellulosic material in alkali. This electrontreatment of the cellulosic material is described in detail, forexample, in DE-A-2,941,624. There it is proposed that pulp be treatedwith 1-30 kGy electron beams before use for viscose production. Thisleads to better processing economics, since electron-treated cellulosicmaterial can be alkalized by means of more dilute caustic soda(concentrations of less than 18%) than is possible in the case ofuntreated cellulose, so that caustic soda consumption distinctlydiminished. At the same time, the carbon disulfide reactivity of thecellulose improves. With correct control of the electron treatment, thechain length of the cellulose can be adjusted to a value at which afurther aging step becomes superfluous. The increased reactivity opensup the prospect of reducing the consumption of carbon disulfide.

The alkali cellulose is then reacted with carbon disulfide. Thisxanthation turns the alkali cellulose into a cellulose xanthate. Thereaction typically involves 3 hours at temperatures of about 25 to 30°C., and a carbon disulfide quantity which typically amounts to 28 to 32%by weight of the cellulosic material present in the alkali cellulose.The resulting cellulose xanthate is obtained as a crumbly yellowishorange mass.

The cellulose xanthate is subsequently dissolved in dilute caustic soda.It is this solution of cellulose xanthate in caustic soda which is knownas viscose.

The viscose is allowed to stand at about ambient temperature for severalhours to ripen. During this time, the xanthate groups become distributedalong the cellulose chains. During the ripening time, other processsteps such as filtration and deaeration are carried out as well.

To produce shaped structures, for example fiber, the ripened viscose isforced through small orifices into a spin bath. The spin bath istypically acid in character and customarily comprises sulfuric acid(about 10%), sodium sulfate (about 20%) and also small amounts of zincsulfate (about 1%). The spin bath coagulates the viscose to form fiber.At the same time as the coagulation, the cellulose is regenerated fromthe cellulose xanthate. The regenerated fibrous cellulose product isthen cut, washed to remove concomitants and impurities, and dried.

The properties of the products can be influenced within limits throughvariation of the process parameters of viscose production and ofspinning.

A very significant cost factor for making products composed of viscoseis the consumption of chemicals such as carbon disulfide, caustic sodaand sulfuric acid. However, there are technical limits to reducing theconsumption of chemicals. If the amount of carbon disulfide and/orcaustic soda used drops below certain limits, the filterability of theviscose deteriorates, but good viscose filterability is an absolute mustfor the entire process to be economically feasible.

In the past, there have been repeated attempts to improve the economicsof the viscose process by reducing the consumption of chemicals. Onepossible way is to use electron-treated pulp.

Although electron-treated pulp may be used for the entire production, aproduction plant may be faced with the need to use normal, untreatedpulp as well to some extent. As explained above, alkali celluloseprepared from electron-treated cellulose no longer has to be aged, sothat it cannot be mixed with alkali cellulose prepared from untreatedcellulose until after the aging thereof.

In certain cases, it can be useful or necessary to keep the two types ofalkali cellulose product separate through to the finished product. Inboth cases, however, it is economically advantageous to carry out thealkalization with a common caustic system. Not only is the equipmentsimpler, but the simpler processing procedure is another point in favorof this solution. However, if electron-treated pulp is alkalized usingthe same caustic previously used to alkalize the untreated pulp,incompatibility becomes apparent as a dramatic deterioration in thefilterability of the viscose produced. This is documented by Example 1described below.

There is therefore a need for a process for producing viscose in whichelectron-treated cellulosic material can be alkalized using a causticpreviously used in the alkalization of untreated cellulose and whichdoes not give rise to the above-described incompatibility. The use of acommon caustic system for both sources of raw material constitutes afurther improvement in the economics of viscose production, sinceseparate handling of the alkalizing caustic can be dispensed with.

SUMMARY OF THE INVENTION

It has now been found that, surprisingly, the above-describedincompatibility of the caustic system can be avoided if the causticobtained from the alkalization of the untreated cellulose is subjectedto a treatment whereby dispersed solids present in the caustic aresubstantially removed before use in the alkalization of electron-treatedcellulose. A viscose thus produced has an acceptable filter value forindustrial production.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention accordingly provides a process for producingviscose, which comprises the steps of:

a) dispersing electron-treated cellulosic material in an aqueoussolution of caustic soda in a conventional manner,

b) removing excess caustic soda from the resulting alkali cellulose in aconventional manner,

c) xanthating the alkali cellulose obtained as per step b) by reactionwith carbon disulfide in a conventional manner,

d) dissolving the cellulose xanthate formed as per step c) in an aqueoussolution of a lye in a conventional manner,

wherein the caustic soda used in step a) was previously used in thealkalization of electron-untreated cellulose and treated in such a waythat the level of dispersed solids in the caustic does not exceed 0.16g/l.

Dispersed solids for the purposes of the present invention areundissolved impurities present in the caustic soda which result from thealkalization of electron-untreated cellulose. The caustic sodapreviously used in the alkalization of electron-untreated cellulose isalso known as plant caustic. It, if necessary, is adjusted before stepa) to the requisite concentration by dilution or addition of freshcaustic soda. The dispersed solids in the plant caustic are determinedby filtering a defined quantity of the plant caustic through a G3 glassfrit and then effecting a quantitative determination of the filter cake.

The electron-treated cellulosic material used as per step a) ispreferably a cellulosic material which has been treated with an electronbeam dose of 1 to 30 kGy. The electron-treated cellulose preferablycomprises at least 85% by weight, especially 90 to 98% by weight, ofalpha-cellulose.

The electron-treated cellulosic material is alkalized by reaction withan aqueous solution of caustic soda previously used in the alkalizationof electron-untreated cellulose. What is essential is that said aqueoussolution of caustic soda was subjected to a treatment so that the levelof dispersed solids in the caustic soda does not exceed 0.16 g/l. Thiscan be accomplished by working up the caustic using techniques known toone skilled in the art.

Suitable measures include, for example, filtration, centrifugation,sedimentation or equivalent solid/liquid separation techniques. Theconcentration of the caustic soda is customarily between 14 and 20% byweight of NaOH, especially between 15 and 17% by weight of NaOH. Theconcentration of the aqueous solution of caustic soda used is adjustedby addition of water and/or caustic soda.

Then excess caustic soda is removed. This can be accomplished, forexample, by pressing, centrifugation or other suitable measures. Thealkali cellulose present as per step b) customarily comprises 28 to 38%by weight, preferably between 30 and 35% by weight, of cellulose andbetween 12 and 18% by weight, preferably 14 to 17% by weight, of causticsoda (NaOH). The degree of polymerization (DP) of the alkali celluloseis between 200 and 500, preferably 250 and 400. The degree ofpolymerization is determined by viscometry of a solution of alkalicellulose in an aqueous metal/amine complex as per TAPPI Standard T 206.

The xanthation of the alkali cellulose (step c) is effected by reactionwith 15 to 35%, preferably 15 to 25%, of carbon disulfide. Thexanthation customarily takes place at room temperature or slightlyelevated temperature.

The cellulose xanthate formed is then dissolved with an aqueous lye. Theaqueous lye used is preferably caustic soda. The resulting basicsolution of the cellulose xanthate is known as viscose. It customarilycomprises 7 to 11% by weight, preferably 8 to 10% by weight, ofcellulose and 3 to 8% by weight, preferably 3.8 to 5% by weight, ofNaOH. In addition to these constituents, the viscose may furthercomprise additives to improve the spinnability of the viscose, forexample. Especially glycol like and aminic modifiers may be mentionedhere.

The viscose can then be used in a conventional manner to produce shapedstructures, for example fiber or film, which likewise forms part of thesubject-matter of the present invention. Here the cellulose isregenerated from the cellulose xanthate.

The viscose is subjected to a filtration prior to the shaping process.The filtration will reveal the above-described incompatibility in thecase of the use of caustic sodas whose level of dispersed solids is morethan 0.16 g/l, i.e., the viscose has a filter value of more than 700.

The Examples which follow illustrate the present invention:

EXAMPLE 1

300 g of electron-treated viscose pulp were steeped with alkalizingcaustic (200 g/l of NaOH) from an industrial-scale production plant (useof untreated cellulose/plant caustic) and then adjusted to a compositionof 32.5% of cellulose and 15.2% of NaOH by centrifugation. The resultingalkali cellulose was xanthated with 28% of carbon disulfide (based onthe cellulose content). Dissolving with dilute caustic soda resulted inthe viscose properties shown hereinafter in the “Plant caustic” column.

A comparative viscose was produced in the same way using freshly made-upalkalizing caustic of the same concentration. The properties of thisviscose are shown in the “Fresh caustic” column of TABLE 1 below.

TABLE 1 Plant caustic Fresh caustic Cellulose in viscose, in % 9.0 9.0NaOH in viscose, % 4.0 4.0 Filter value 2040 152

The filter value used here to characterize the filterability ispredictive of the following plant filterability shown in TABLE 2 below:

TABLE 2 Filter value Plant filterability <400 very good 400-500 good500-600 moderate 600-700 poor >700 unfilterable

The results show that the use of a plant alkalizing caustic, i.e., of acaustic previously used in the alkalization of untreated cellulose andsubsequently not filtered, leads to an unfilterable viscose. The processof the present invention provides a solution for conjoint utilization ofone alkalizing system for untreated and electron-treated pulp.

It was found, then, that, surprisingly, the abovementionedincompatibility documented in Example 1 is attributable to dispersedsolids in the plant alkalizing caustic, which are removable, for exampleby mechanical filtration. If the plant alkalizing caustic isincreasingly closely filtered before use for alkalizing electron-treatedpulp, increasingly better filtration properties are obtained for theviscose produced, as shown by the series of runs of Example 2.

EXAMPLE 2

For each run, 300 g of electron-treated viscose pulp were steeped withalkalizing caustic (190 g of NaOH /l) from an industrial-scaleproduction plant (use of untreated cellulose/plant caustic) and filteredto different degrees before use, so that different concentrations ofdispersed solids remain in the caustic. The resulting alkali cellulosewas then centrifuged to a composition of 31.8-32.2% of cellulose and16.2-16.4% of NaOH and xanthated with 22% of carbon disulfide (based onthe cellulose content). Dissolving with dilute caustic soda producedviscoses having a cellulose content of 8.9-9.0% and an NaOH content of4.0-4.1%. The degree of filtration was measured in terms of theconcentration of admixtures still removable from the alkalizing causticusing a G3 glass frit. The results are shown in TABLE 3 below.

TABLE 3 Admixtures in alkalizing Run No. caustic (g/l) Filter value 11.25 5680 2 0.46 945 3 0.28 739 4 0.16 498 5 0.03 247 6 0.008 193 70.002 196 8 0.000 170

The filterability of the viscoses can be judged against the scalereported in Example 1.

The results show that, when the caustic used for alkalizing comprises adispersed solids content of 0.28 g/l, the viscose produced therewith isno longer filterable on an industrial scale. Good filtration propertiesare only achievable when the level of filter-removable solids does notexceed 0.16 g/l.

What is claimed is:
 1. A process for producing viscose, which comprisesthe steps of: a) treating cellulosic material with electrons tomanufacture an electron-treated cellulosic material; b) dispersing theelectron-treated cellulosic material in an aqueous solution of causticsoda to produce an alkali cellulose; c) removing excess caustic sodafrom the alkali cellulose; d) xanthating the alkali cellulose obtainedin step c) by reaction with carbon disulfide to produce cellulosexanthate; and e) dissolving the cellulose xanthate formed in step d) inan aqueous solution of a lye, wherein the caustic soda used in step bwas previously used in the alkalization of electron-untreated celluloseand treated such that the level of dispersed solids in the caustic sodadoes not exceed 0.16 g/l.
 2. The process of claim 1, wherein the causticsoda used in step b) has a concentration that corresponds to 14 to 20%by weight of NaOH.
 3. The process of claim 1, wherein the level ofdispersed solids in the caustic soda used in step b) is determined byfiltration.
 4. The process of claim 1, wherein the caustic soda used instep b) comprises not more than 0.03 g/l of dispersed solids.
 5. Theprocess of claim 1, wherein the caustic soda used in step b) is treatedby at least one of filtration and centrifugation.
 6. A process forproducing viscose, which comprises the steps of: a) providing causticsoda previously used in the alkalization of electron-untreatedcellulose, the caustic soda having a level of dispersed solids notexceeding 0.16 g/l; b) treating cellulosic material with electrons tomanufacture an electron-treated cellulosic material; c) dispersing theelectron-treated cellulosic material in an aqueous solution of thecaustic soda to produce an alkali cellulose; d) removing excess causticsoda from the alkali cellulose; e) reacting the alkali celluloseobtained in step d) with carbon disulfide to produce cellulose xanthate;and f) dissolving the cellulose xanthate formed in step e) in an aqueoussolution.
 7. The process of claim 6, wherein the caustic soda used instep c) has a concentration that corresponds to 14 to 20% by weight ofNaOH. 8.The process of claim 6, wherein the level of the dispersedsolids in the caustic soda is determined by filtration.
 9. The processof claim 6, wherein the caustic soda provided in step a) comprises notmore than 0.03 g/l of dispersed solids.
 10. The process of claim 6,wherein prior to step a) the caustic soda previously used in thealkalization of electron-untreated cellulose is treated by at least oneof filtration and centrifugation.
 11. The process of claim 6, whereinthe aqueous solution used in step f) comprises a lye.
 12. A process forproducing viscose, which comprises the steps of: a) alkalizingelectron-untreated cellulose in a caustic system; b) treating causticsoda previously used in step a) such that the caustic soda has a levelof dispersed solids not exceeding 0.16 g/l; c) treating cellulosicmaterial with electrons to manufacture an electron-treated cellulosicmaterial; d) dispersing the electron-treated cellulosic material in anaqueous solution of the caustic soda in the caustic system to produce analkali cellulose; e) removing excess caustic soda from the alkalicellulose; f) reacting the alkali cellulose obtained in step e) withcarbon disulfide to produce cellulose xanthate; and g) dissolving thecellulose xanthate formed in step f) in an aqueous solution.
 13. Theprocess of claim 12, wherein the caustic soda used in step d) has aconcentration that corresponds to 14 to 20% by weight of NaOH.
 14. Theprocess of claim 12, wherein the level of the dispersed solids in thecaustic soda is determined by filtration.
 15. The process of claim 12,wherein the caustic soda used in step d) comprises not more than 0.03g/l of dispersed solids.
 16. The process of claim 12, wherein step b)comprises treating the caustic soda previously used in step a) by atleast one of filtration and centrifugation.
 17. The process of claim 1,wherein the aqueous solution used in step g) comprises a lye.